By Jack Smith
To help meet available fault current labeling requirements, use an electrical test instrument such as this power quality analyzer.
The effective date of the latest version (the 2011 edition) of “NFPA 70: National Electric Code” (NEC) was August 25, 2010. It was published in September 2010.
The NEC is the authoritative electrical installation code in the United States. However, there’s no consistency in how it’s enforced. The NEC must be adopted into law by individual states and local jurisdictions. The timing for NEC adoption varies among states. To add to the confusion, eight states have “local adoption,” which means that different municipalities or regions within the state adopt the NEC independently.
For example, Illinois is one of the eight states that allow local adoption of the NEC. Of the 16 regions, nine follow the 2008 edition, two follow the 2005 edition, and four follow the 2002 edition. However, instead of the NEC, Chicago has its own code: the Chicago Electrical Code. Its latest version is the 2010 edition.
As of December 2011, when the National Fire Protection Association (NFPA) updated its “NEC Adoption by State” map, only two states - Connecticut and Missouri - use the 2005 edition and 25 states use the 2008 edition. Only 15 states have adopted the 2011 edition of the NEC. The result is different NEC versions are enforced in different parts of the US and even within some states.
The NEC is developed under the authority of the NFPA using an open, consensus-based process. As with most of the NFPA codes and standards, the development process is complex. Explaining it is well beyond the scope of this column, but it’s described in detail at www.nfpa.org. There were more than 500 changes during its latest revision cycle. Many of these are minor wording changes intended to clarify specific points. However, some changes impact the scope of the NEC significantly.
New requirement: available fault current labeling
One of the new sections - “110.24 Available Fault Current” - requires certain pieces of equipment to be marked with the amount of fault current available to that equipment. It also requires this marking to be updated if electrical system modifications occur that affect the equipment.
Part A of 110.24 states, “Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved.”
Before Section 110.24 was added, it was up to the installers to ensure that electrical equipment interrupting ratings were sufficient for the available fault current connected to that equipment. It was up to the inspectors for the authorities having jurisdiction to enforce it. Installers and inspectors had to determine the available fault current - usually available from the utility - and then compare that value with the appropriate equipment ratings.
These “appropriate equipment ratings” include interrupting rating and short circuit current rating (SCCR). The “official” NEC definition of SCCR is, “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.”
In other words, SCCR is the maximum level of short-circuit current a component or equipment can withstand without sustaining damage or creating hazardous operating conditions. SCCR is not a component’s interrupting rating - these two terms must not be confused. Interrupting ratings are applicable only to the devices that are intended to interrupt current: fuses or circuit breakers. They’re not applicable to the assembly or panel in which the fuses or breakers are installed or the electrical components they protect.
It’s also important to understand the difference between interrupting rating and trip setting or overload. The two types of overcurrent are short circuits and overloads. Overloads are the most frequently occurring electrical problem. Generally, overloads are less than 10 times the rated circuit current, while short circuits are more than 10 times that amount. For the record, circuit breakers trip and fuses open in response to both overloads and short circuits.
Making modifications means making marking changes
Part B of Section 110.24 states, “When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) must be adjusted to reflect the new level of maximum available fault current.”
In other words, if changes are made that affect available fault current, the label must show the new rating as well as the date the new rating was determined. The “Exception” to Section 110.24 in the 2011 NEC states, “Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.”
This exception gives most large industrial facilities the credit for ensuring that they update electrical equipment services regularly and that required equipment labeling changes will be made accordingly. Keeping facility electrical systems and electrical equipment up to date requires high quality electrical test instruments such as portable oscilloscopes, power quality analyzers, clamp meters, and digital multimeters like those available from Fluke.
Electrical characteristics may change as facility electrical systems age. Utilities may replace aging transformers with more efficient lower impedance units. Available fault current and equipment ratings must be re-evaluated when utilities change transformers or when facilities install emergency or standby power systems.
This new available fault current labeling requirement is applicable to equipment ratings only. It is not to be used in arc-flash hazard analyses.
Until next time, keep standing on “Solid Ground.”